Hey there,
Read, research and read some more Something like this (although dated) might give some ideas: How to Build: All-in-One PC | PC Gamer
A few years ago, yes you'd have been able to source parts off the shelf to build yourself an AIO PC but nowadays the option for an AIO has moved back to the system integrators or big wig builders/brands. One such example is here though you are compromising on some aspects of the build when you move to an AIO like lack of a beefy storage setup or upgrade path, as well as a non discrete GPU layout.